Design of a compact off-axis freeform three-mirror system in a circular configuration

A compact off-axis three-mirror system, especially with a wide field of view and small f-number, continues to present challenges in the optical design field. To design a compact off-axis three-mirror system based on a freeform surface, an optimization method with a circular configuration and four parameters is proposed. In the proposed method, the f-number and angles of mirrors are combined to optimize and achieve high-quality imaging, which means the modulation transfer function is close to the diffraction limit. To evaluate the performance of the proposed method, four design examples with different fields of view (4^∘×4^∘, 6^∘×6^∘) and f-numbers (2, 1.75) were created. The results showed that the compact off-axis three-mirror system based on a freeform surface can be designed with a wide field of view, small f-number, and high-quality imaging using the proposed method. Unlike other design methods, the surface parameters of mirrors are obtained with only one calculation and are close to the final optimization results, which saves both time and resources for optimization. The findings indicate that the method is accurate and effective for designing compact off-axis three-mirror systems with freeform surfaces.